Electric-field meter having current compensation

ABSTRACT

An electric-field meter provided with a housing, an electrode assembly, a shield assembly, a movement assembly, a position detection assembly, a charge measurement circuit, and a leakage current compensation circuit. The electric-field meter can be characterized as a field mill, an induction voltmeter, an electrostatic fluxmeter or an agrimeter. The electrode assembly is selectively exposed to the electric field. The shield assembly alternately covers and exposes the electrode assembly to the electric field. The movement assembly has a source of motive force and a linkage operably connected to one of the shield assembly and the electrode assembly for alternately covering and exposing the electrode assembly to the electric field. The charge measurement circuit receives charge on the electrode assembly. The charge measurement circuit provides a charge detection signal indicative of the charge induced on the electrode assembly as the electrode assembly is selectively exposed to the electric field. A current compensation circuit is provided for offsetting an average leakage current at the input of the charge measurement circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation-in-part of U.S. Ser. No.10/094,942, filed Mar. 14, 2002, now U.S. Pat. No. 6,984,971 B1 issuedJan. 10, 2006 which claims priority to the provisional patentapplication identified by U.S. Ser. No. 60/275,763, filed on Mar. 14,2001, the entire contents of U.S. Ser. Nos. 10/094,942 and 60/275,763are hereby expressly incorporated in their entirety by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

Portions of the invention may have been funded under NSF GrantATM-9724594.

BACKGROUND OF THE INVENTION

Lightning is a well known natural hazard. Every year in the UnitedStates, lightning kills approximately one hundred people and injures onthe order of a thousand. Approximately 50 million cloud-to-groundlightning discharges occur each year in the U.S. Damage to equipment anddisruption of commercial and industrial operations is measured inbillions of dollars. For these reasons, and for many safety-related andequipment protection purposes, it is desirable to provide objectiveinformation about impending electrical storms, active thunderstorms andexpiring thunderstorms.

Instruments known as electric-field meters are currently used to measureatmospheric electric fields at the surface of the earth. Almost all suchelectric-field meters are based on a design originally developed byC.T.R. Wilson in the early part of the 20th Century. One of the priorart electric-field meters developed by Wilson used a flat circular metalplate mounted flush with, but well insulated from the ground. The flatplate was connected to a gold-leaf electrometer. To make a measurementof electric field with this instrument, a grounded cover was placed overthe sensor plate (thus shielding the sensor plate from the ambientelectric field) and a zero for the electrometer was determined. Then thecover was removed, allowing charge to be induced on the plate andcausing a deflection of the electrometer leaves. By means of acalibrated variable capacitor and a power supply, Wilson was able tonull out the induced charge and thereby determine the electric field atthe sensor plate when it was uncovered. All mechanized electric-fieldmeters that followed have been, essentially and simply, variations withvarying degrees of automation of the basic concepts employed by Wilson.

Mechanized electrical field meters have been employed for atmosphericresearch and thunderstorm warning for about seventy years. Mechanizedfield meters have been used as stand-alone instruments and in networksin which multiple individual sensors are installed some distance aparton the surface of the Earth to give measurements of electric fields overa wide area.

Multiple field meters in a network have been employed at the NASAKennedy Space Center for more than 20 years as one component of adecision support system to inform official judgement as to propriety offueling operations, launch, etc. Single field meters are employed athigh-risk installations such as armament caches, etc. The cost ofcommercial field meters currently available is high, they have greatelectrical power requirements, and they usually need frequent preventiveand periodic maintenance. These disadvantages preclude widespreadapplication of commercially available field meters.

More specifically, the prior art electric-field meters suffer from atleast three problems which make their wide spread use generally toocostly. These three factors are relatively high power consumption,difficult calibration procedure, and stringent requirements for frequentmaintenance. For example, on all high input impedance electric-fieldmeters it is necessary to clean the insulators and/or the electrodes ofthe insulated sensing electrode assembly periodically. Cleaning isnecessary because when the insulators become covered with films of dust,moisture and salt spray, conductive paths can form, defeating thepurpose of the insulators. Over time, the sensing electrode assemblyalso becomes covered with a film of dust and salt spray. In the priorart, the cleaning operation is difficult because the prior artelectric-field meters require extensive and complex disassembly of theinstrument to remove electrodes and thereby clean insulators andelectrodes. The disassembly of the prior art electric-field meters forelectrode and insulator cleaning thus requires a highly skilledtechnician adding considerably to the on-going expenses associated withthe electric-field meter.

Commercial field meters typically consume tens to hundreds of watts ofelectrical power. Such high power consumption precludes or discouragesapplication of commercial field meters on most of the existing remote,solar-powered weather stations where electrical power is severelylimited.

Commercial field meters, when mounted for practical use in elevatedconfigurations, e.g., above ground, on top of buildings, on weatherstation masts or poles, for which the electric field enhancement factoris unknown must be properly adjusted to compensate for the mechanicallyincreased gain due to the mounting. Typically this correction isperformed by changing the value of a resistor or by adjusting a variableresistor inside the instrument to effect a reduction of the electricalgain of the instrument by the same factor that the gain is enhanced bythe mounting arrangement. This gain adjustment process typicallyinvolves disassembly of the instrument to gain access to electroniccomponents. This process also typically involves a skilled technicianand involves risks of opening and improperly closing sealed enclosuresin the field.

Prior-art field meters suffer from two types of uncorrected errors thatchange with time, temperature, humidity and atmospheric pollutants.Typical instruments that predate the present invention have azero-signal output (defined as the output value of the field meter withan imposed electric field of zero) that is typically set duringmanufacture but which subsequently changes in an unknown way with useand time. Because valuable information about atmospheric electricalconditions can be obtained around zero and at the zero-crossing, i.e.,when the electric field reverses polarity, there is a significantadvantage in having a zero-signal reading that is known with confidencethroughout the operating life of the instrument.

Prior-art field meters also suffer from variations in leakage current atthe charge-amplifier input due to conduction across insulatorsassociated with the sense electrode and the circuitry used for chargemeasurement. For prior-art field meters at the place and time ofmanufacture, the average leakage current at the charge-amplifier inputis typically negligible but it can worsen overtime with variousatmospheric conditions. The average leakage current in prior-art fieldmeters is an unknown variable that can degrade an instrument to a stateof improper operation without warning. Uncorrected changes in averageleakage currents may cause measurement errors in the electric field,possibly leading to improper assessment of atmospheric electricalthreats.

Field meters that suffer from unknown and uncorrected zero offsets andaverage leakage currents do not always provide information of highquality over long periods of use and such field meters typically requirelabor intensive testing, adjusting and cleaning at times that have to bedetermined empirically. Here we teach methods for making field metersthat measure and correct zero-signal offset errors and errors due toleakage current at the charge-amplifier input as part of eachmeasurement cycle so that every measurement made and reported is of highquality.

Thus, a need exists in the art for an electric-field meter with lowoperating power requirements, ease of installation and fieldcalibration, minimal on-going maintenance expenses, and continuous andautomatic error detection and correction. It is to such an improvedelectric-field meter that the present invention is directed.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a side elevational view of an electric-field meter constructedin accordance with the present invention, and installed in typicalfashion.

FIG. 2 is a block diagram of the electric-field meter when theelectric-field meter is characterized as a field mill.

FIG. 2A is a block diagram of the electric-field meter when theelectric-field meter is characterized as an induction voltmeter.

FIG. 2B is a block diagram of the electric-field meter when theelectric-field meter is characterized as an electrostatic fluxmeter.

FIG. 2C is a block diagram of the electric-field meter when theelectric-field meter is characterized as an agrimeter.

FIG. 3 is a perspective view showing a lower portion of one embodimentof the electric-field meter.

FIG. 4 is a bottom plan view of the electric-field meter depicted inFIG. 3.

FIGS. 5 and 6 are diagrammatic views of other embodiments of theelectric-field meter wherein the electric-field meter is provided withan adjustable rod or plate for mechanically adjusting the gain of theelectric-field meter.

FIGS. 7 and 8 are diagrammatic views of other embodiments of theelectric-field meter wherein the electric-field meter is adjustablerelative to a mounting device for mechanically adjusting the gain of theelectric-field meter.

FIG. 9 is a perspective view of another embodiment of a mechanical gainadjustment assembly constructed in accordance with the presentinvention.

FIG. 10 is an exploded, side elevational view of an electrode assemblyconstructed in accordance with the present invention.

FIG. 11 is a perspective view of one embodiment of the electric-fieldmeter having a housing removed.

FIG. 12 is a schematic diagram of the electric-field meter depicted inFIG. 1 wherein a position detection assembly is shown in more detail.

FIG. 13 is a top plan view of an electrical contact assembly constructedin accordance with the present invention.

FIG. 14 is a side elevational view of the electrical contact assemblydepicted in FIG. 13.

FIG. 15 is a fragmental view of a second version of an electricalcontact assembly constructed in accordance with the present invention.

FIG. 16 is a side elevational view of a third version of an electricalcontact assembly constructed in accordance with the present invention.

FIG. 17 is a top plan view of a fourth version of an electrical contactassembly constructed in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to improvements in electric-fieldmeters to allow more cost effective and reliable monitoring ofatmospheric electrification at single or multiple remote stations. Theelectric-field meter of the present invention exploits the physicalprinciple that charge is induced on a conductor placed in an electricfield. By alternately covering and exposing a conducting and properlyinsulated electrode assembly, induced charge flows back and forth to thesuccessively exposed, then unexposed, electrode assembly throughcharge-sensing electronic circuits. The electronic signal produced isindicative of (e.g., proportional to) the applied electric field at theelectrode assembly when exposed. Thus, the electric-field meters can beused for measuring a magnitude and/or a polarity of a steady or changingelectric field.

The electric-field meters of the present invention can be used eitherindividually or as a part of a network for research or for widespreadmonitoring and warning of impending electrical storms at the followingexemplary locations and activities: a golf course, an airport, a marina,an agricultural operation, an offshore oil rig, a hiking trail, anoutdoor stadium (football, soccer, baseball, track and field), a themepark, a swimming pool, explosive and munition handling, or any otheroutdoor operation, such as loading of freight on an aircraft, fueling orthe like. When more than one of the electric-field meters of the presentinvention are utilized, data from separate electric-field metersarranged in a network or grid can be taken to build contours of electricfield at ground level as a function of time for operational purposes(hazard warnings, all-clear notice, or the like) and for meteorologicalresearch. The electric-field meter can be used for measurement ofatmospheric electric fields for a variety of purposes, such asthunderstorm research, early warning of impending lightning, airpollution monitoring, industrial and process control, industrial safety,high-voltage laboratories, physics experiments and educationaldemonstrations, and intrusion detection and alarm systems.

Some of the improvements to the electric-field meters in accordance withthe present invention can be characterized according to the followingclasses: mechanical gain adjustment assemblies, insulated electrodeassemblies, rotating electrical contact assemblies, and in someembodiments, elimination of the need for such assemblies, and positiondetection assemblies. Each of these improvements can be usedindividually or in combination with other improvements discussed hereinto achieve improved performance in comparison to prior artelectric-field meters.

The mechanical gain adjustment assemblies and the insulated electrodeassemblies 1) permit less expensive and more reliable electric-fieldmeters to be constructed, and 2) simplify installation procedures forelectric-field meters.

The electrical contact assemblies of the present invention, or theelimination of the need for them in some embodiments of the presentinvention, improve the reliability of electric-field meters and alsoimprove the grounding of or signal conduction from rotating or movingparts thereby improving the signal-to noise ratio of the output of theelectric-field meter. The position detection assemblies greatly reduceelectrical power consumption as well as complexity of setup andmaintenance of electric-field meters constructed in accordance with thepresent invention relative to the prior art electric-field meters.

Various devices are described herein for implementing each of theimprovements discussed above. However, it should be understood that eachof the improvements can be implemented in various manners so long as thefunctions described herein are achieved.

Referring now to FIG. 1, shown therein and designated by a referencenumeral 10 is an electric-field meter constructed in accordance with thepresent invention. In one preferred embodiment, the electric-field meter10 requires power of only six hundred (600) milliwatts or less. Theelectric-field meter 10 can be provided with a complete internalbattery-powered sub-system that enables the electric-field meter 10 tooperate for extended periods of time while remaining independent ofexternal power sources. Alternatively, the electric-field meter 10 maybe powered by a small, inexpensive solar panel that can provide adequaterecharge of sealed, internal lead-acid (or other types) batteries, forexample, for unattended long term operation. The prior artelectric-field meters discussed in the background of the presentinvention typically require tens to hundreds of watts. Thus, theelectric-field meter 10 is a substantial improvement over the prior artelectric-field meters.

As shown in FIG. 1, the electric-field meter 10 can be mounted andsupported by a mounting device 12. The electric-field meter 10 ispreferably mounted on the mounting device 12 in an “inverted” positionwherein an electrode assembly of the electric-field meter faces theground so as to provide some protection for the electrode assembly fromthe elements, such as rain, snow, hail and ice. However, theelectric-field meter 10 can be mounted to face upwards.

Mounting methods, such as the mounting device 12 for supporting theelectric-field meter 10 are well known in the art. Thus, no morecomments are deemed necessary to teach one of ordinary skill in the arthow to construct and/or use a mounting device, such as the mountingdevice 12. The present invention is not limited to any particular typeof mounting device.

Referring now to FIG. 2, shown therein, in block diagram form, is onepreferred embodiment of the electric-field meter 10. The electric-fieldmeter 10 described hereinafter is also known as a “field mill”. However,as will be discussed in more detail below with reference to FIGS. 2A, 2Band 2C it should be understood that the present invention is equallyadapted to other electromechanical electric-field sensing instruments,such as electrostatic fluxmeters (FIG. 2B) induction voltmeters (FIG.2A) and agrimeters (FIG. 2C), which share the common problems ofcalibration, making proper reliable contact with moving or rotatingconductors, achieving and maintaining proper insulation of signalcarrying conductors, correcting for signal modification due to non-idealmounting configuration and detecting the position of moving conductorsfor the purpose of synchronous rectification.

In general, the electric-field meter 10 shown in FIG. 2 is provided withone or more electrode assembly 26, a shield assembly 28, a movementassembly 30, one or more position detection assembly 32, a chargemeasurement circuit 34, and a microcontroller 36 with ananalog-to-digital converter.

In the field-mill implementation, the electrode assembly 26 ismaintained in a stationary position and is selectively exposed to anelectric field 38. Charge is induced on the electrode assembly 26 whenthe electrode assembly 26 is exposed to the electric field 38. Chargepasses to ground from the electrode assembly 26 when the electrodeassembly 26 is shielded from the electric field 38. The electric-fieldmeter 10 can be provided with one or a plurality of electrode assemblies26 preferably mounted in a symmetrical or evenly spaced relationship.For example, as shown in FIG. 4, the electric-field meter 10 has fourelectrode assemblies 26 with each of the electrode assemblies 26 mounted90 degrees apart and in which opposing pairs are electrically common.

When the electric-field meter is a “field mill”, the shield assembly 28is maintained at a ground reference potential 40 and is movable withrespect to the electrode assembly 26 through a predetermined path foralternately covering and exposing the electrode assembly 26 to theelectric field 38. In this example, the shield assembly 28 is providedwith one or more vanes 29 (FIG. 4), but the shield assembly 28 can takewhatever form is needed for covering and exposing the electrode assembly26.

The movement assembly 30 has a linkage 42 operably connected to theshield assembly 28 for moving the shield assembly 28 with respect to theelectrode assembly 26. The linkage 42 can be a mechanical linkage, suchas the shaft of a motor. The movement of the shield assembly 28 by themovement assembly 30 alternately covers and exposes the electrodeassembly 26 to the electric field 38.

The movement assembly 30 can move the shield assembly 28 in a rotarymanner, a reciprocating manner, or an oscillatory manner. The rotarymanner of movement of the shield assembly 28 refers to movement of theshield assembly 28 in only one direction through a closed pathencompassing at least 360 degrees. The movement assembly 30 providingthe rotary manner of movement of the shield assembly 28 can be a steppermotor, a DC brushed motor, a DC brushless motor, a DC servo motor, asolenoid, a rotary actuator, or a clockwork-spring-like mechanism, orany of a variety of single or multiphase AC motors.

The reciprocating manner of movement refers to linear or rotary motioninvolving back and forth movement, up and down movement or the like. Inthe case of rotary reciprocating motion, the shield assembly 28 may movethrough any angle desired up to the limit imposed by an electricalcontact assembly discussed herein below with reference to FIGS. 13–17.The reciprocating manner of movement can be implemented with a steppermotor, a DC brushed motor, a DC brushless motor, a solenoid(s), a rotaryactuator, a clockwork-spring-like mechanism, a linear actuator, a DCmotor with servo-control, a mechanical spring with an assist from anelectromechanical device, an air motor, or a fluid drive. Theoscillatory manner of movement refers to the shield assembly 28 andmovement assembly 30 being a driven mechanical oscillator, i.e., amechanical system having a natural frequency of oscillation.

The stepper motor provides many advantages over other ways of moving theshield assembly 28. That is, the stepper motor allows for (1) low-noisebrushless DC operation and allows for rapid, repeatable motion of theshield assembly 28; (2) electromagnetic braking of the shield assembly28 and precise repeatable repositioning of the shield assembly 28; (3)very low average power consumption because the stepping motor coils canbe de-energized after a move is complete; (4) complete symmetry ofcovering and uncovering “moves” that can be used to advantage forimproved signal processing; and (5) control with only two I/O lines fromthe microcontroller 36 thereby providing economy in hardware andsoftware. The stepper motor can give a reduction in number of parts andthereby a reduction in cost to manufacture and cost to maintain and anincrease in reliability.

In the reciprocating and oscillating manners of movement, thereoptionally may be a pause (hereinafter referred to herein as “deadtime”) between movement of the shield assembly such that 1) the shieldassembly 28 is not moving and 2) the shield assembly 28 is not coveringthe electrode assembly 26. dE/dt signals can optionally be received bythe charge measurement circuit 34 and monitored during dead time whenthe electrode assembly 26 is exposed to electric-field change signals.This in effect allows the electric-field meter 10 to function as anelectric-field-change meter, which monitors the environment for changesof electric-field during dead time. Using the electric-field meter 10during dead time as the electric-field-change meter makes theelectric-field meter 10 responsive to both electric field changes causedby lightning, and the relatively slow variations in electric-fieldchanges associated with growth and decay of as well as advection chargeaccumulations in clouds. Both of these functions are useful if theelectric-field meter 10 is being used for monitoring thunderstorms. Forexample, the electric-field meter 10 can be used for countinglightning-like field changes, watching for lightning and other largefield changes, and other conditions which are indicative of athunderstorm threat.

The position detection assembly 32 monitors the position of at least aportion of the shield assembly 28 and outputs a shield positiondetection signal. The shield position detection signal indicates theportion of the shield assembly 28 for typically determining the coveringor uncovering of the electrode assembly 26 by the shield assembly 28.The charge measurement circuit 34 is connected to the electrode assembly26 and produces a charge measurement signal indicative of the chargeaccumulated, or, released by the electrode assembly 26. The chargemeasurement circuit 34 can be implemented with a charge amplifier, orwith a voltage amplifier that measures the voltage drop across aresistor connected between the electrode assembly 26 and the groundreference potential 40.

The microcontroller 36 receives the shield position detection signal andthe charge measurement signal and determines the sign and magnitude ofthe electric field 38 based on the shield position detection signal andthe charge measurement signal.

The microcontroller 36 outputs a signal indicative of the electric field38 to a meter, a computer system, or a dedicated display located locallyor at a central monitoring station. The signal output by themicrocontroller 36 can be either an analog or a digital signal, or both.Optionally, the electric-field meter 10 can be provided with afiber-optic communication link for communicating the signal output fromthe microcontroller 36 to the central monitoring station. The type ofsignal output by the microcontroller 36 can vary widely depending on theapplication. In one preferred embodiment, the output from themicrocontroller 36 is provided as a digital representation of an analogsignal with full-scale equal to plus or minus 2.5 volts whichcorresponds to plus or minus 10 kilovolts per meter of electric field.

When the movement assembly 30 is the stepper motor, the microcontroller36 is typically used to control the stepper motor, and to sample thecharge measurement circuit 34 (in conjunction with an A/D converter) forperforming a sampling of the peak output signal from the chargemeasurement circuit 34 on both covering and uncovering operations. Thisscheme of using an A/D converter to digitize the peak voltage out of thecharge measurement circuit 34 provides a sample and hold function withinfinite hold time. This feature greatly relaxes the requirements forperfect insulators and long amplifier decay times for the chargemeasurement circuit 34, thus reducing maintenance requirements evenfurther, beyond the improvements afforded by the mechanical designs ofthe present invention.

This simple combination of features (moving the shield assembly 28 witha microprocessor-controlled stepper motor detecting position andsampling the peak voltage resulting) replaces the entire synchronousrectifier portion of a conventional field meter. The electric-fieldmeter 10 is thus automatically synchronous. Since the microcontroller 36causes the covering and uncovering motions, the microcontroller 36 knowsat all times the position of the shield assembly 28 so that the outputsignal of the charge measurement circuit 34 can always be sampled at theproper time and thus the proper sign can be assigned to the electricfield measurement.

As shown in FIGS. 2, 2A, 2B, and 2C, the electric-field meter 10 can becharacterized as a field mill (FIG. 2), an induction voltmeter 10 a(FIG. 2A), an electrostatic fluxmeter 10 b (FIG. 2B), or an agrimeter 10c (FIG. 2C). The characterization of the electric-field meter 10 as thefield mill 10, the electrostatic fluxmeter 10 b, the induction voltmeter10 a or the agrimeter 10 c depends on the relative movement andgrounding of the shield assembly 28 and the electrode assembly 26 (aswill be discussed below). The making and using of field mills,electrostatic fluxmeters, induction voltmeters and agrimeters is wellknown in the art. For purposes of brevity, the induction voltmeter 10 a,electrostatic fluxmeter 10 b and agrimeter 10 c will only be shownschematically. However, it should be understood that one skilled in theart will be readily able to make and use the induction voltmeter 10 a,electrostatic fluxmeter 10 b and agrimeter 10 c based on the descriptionset forth in the present patent application. For purposes of brevity,similar components between the electric-field meter 10, the inductionvoltmeter 10 a, electrostatic fluxmeter 10 b and agrimeter 10 c areprovided with similar numerical prefixes, and different alphabeticsuffixes.

The field mill 10 has the movable shield assembly 28 that is held at theground reference potential 40. The movable shield assembly 28alternately exposes and covers the electrode assembly 26 which isconnected to the ground reference potential 40 through the chargemeasurement circuit 34. In early implementations, field mills measuredthe alternating potential difference developed across a resistor ascharge on the electrode assembly 26 passed to and from the groundreference potential 40. More modern implementations use an operationalamplifier in the charge measurement circuit 34 to hold the electrodeassembly 26 near the ground reference potential 40 and to performcharge-to-voltage conversion.

As shown in FIG. 2A, the induction voltmeter 10 a is similar to thefield mill except that the electrode assembly 26 a is alternatelyconnected to the ground reference potential 40 a when exposed and to thecharge measurement circuit 34 a when shielded.

As shown in FIG. 2B, the electrostatic fluxmeter 10 b is provided withan electrode assembly 26 b which is movable through a predeterminedpath, and the shield assembly 28 b is maintained at the ground referencepotential 40 b. The shield assembly 28 b is maintained in a stationaryposition adjacent the predetermined path of the electrode assembly 26 bto alternately cover and expose the electrode assembly 26 b to theelectric field when the electrode assembly 26 b moves relative to theshield assembly 28 b. The electrode assembly 26 b is connected to theground reference potential 40 b through the charge measurement circuit34 b.

As shown in FIG. 2C, the agrimeter 10 c is similar to the electrostaticfluxmeter 10 b except that an electrode assembly 26 c is alternatelyconnected to the ground reference potential 40 c when exposed and to thecharge measurement circuit 34 c when shielded.

Mechanical Gain Adjustment

As shown in FIG. 3, in accordance with the present invention, theelectric-field meter 10 can be provided with a mechanical gainadjustment assembly 50 for adjusting the effective area of the electrodeassembly 26. Typically, the mechanical gain adjustment assembly 50 isutilized for correcting the signal augmentation that results fromelevating and inverting the electric-field meter 10 above the surface ofthe Earth.

For example, it is usually desirable to know the actual electric fieldthat exists at ground level. However, to reduce the effects ofprecipitation splash, mud, dust, insects, plant growth etc., theelectric-field meter 10 is typically mounted on the mounting device 12in the elevated and inverted position, resulting in an increase orenhancement of the actual electric field.

A variety of different embodiments of the mechanical gain adjustmentassembly 50 will be discussed below. In each of the embodiments, themechanical gain adjustment assemblies 50 vary the effective “aperture”of the electric-field meter 10 to mechanically vary, i.e., augment orreduce, the exposure of the electrode assembly 26 to the electric field.The effective aperture can be adjusted manually by an operator who hasknowledge of how much the gain of the electric-field meter 10 needs tobe changed.

Shown in FIG. 3 is a perspective view of a lower portion of oneembodiment of the electric-field meter 10. The electric-field meter 10includes a housing 52 at least partially constructed of a conductivematerial. As shown in FIG. 4, the housing 2 has a lower end 54. Theelectrode assembly 26, and the shield assembly 28 are positioned on thelower end 54.

In this embodiment, the mechanical gain adjustment assembly 50 includesa shroud 60 having an open end 62. The open end 62 of the shroud 60defines an aperture 63 that permits exposure of the insulated electrodeassemblies 26 to the electric field 38. The shroud 60 surrounds thehousing 52 and is affixed to provide lengthwise adjustment between theshroud 60 and the housing 52. The lengthwise adjustment permits the openend 62 of the shroud 60 to extend past the lower end 54 of the housing52 such that the distance between the open end 62 and the electrodeassembly 26 can be adjusted. This adjustment mechanically increases orreduces the electric-field to which the electrode assembly 26 isexposed. The mechanical gain adjustment assembly 50 is also providedwith a securing mechanism 64 for selectively permitting and preventingmovement of the shroud 60 on the housing 52 after a desired position hasbeen set. The securing mechanism 64 can be any device capable ofselectively permitting and preventing movement of the shroud 60 relativeto the housing 52. For example, the securing mechanism 64 can be a bandclamp, screws, bolts, cams or combinations thereof.

Although the mechanical gain adjustment assembly 50 has been shown anddescribed herein as the shroud 60 and the securing mechanism 64, itshould be understood that other manners of constructing the mechanicalgain adjustment assembly 50 are contemplated. For example, themechanical gain adjustment assembly 50 can be constructed of thefollowing components either singularly or in combination: one or moreselectively movable and securable conductive rods or conductive plates66 positioned near the electrode assembly 26 as shown in FIGS. 5 and 6;an iris or shutter assembly having a plurality of movable vanes forchanging the size of an aperture (not shown) providing access to theinsulated electrode assemblies 26 and the shield assembly 28; a pivotalconnection 68 between the electric-field meter 10 and the mountingdevice 12 for moving the electrode assembly 26 closer to a grounded,conducting object, such as a mounting mast or stanchion as shown in FIG.7; or a sliding or telescoping mechanism 70 permitting adjustment of theheight of the electric-field meter 10 relative to the ground as shown inFIG. 8.

Referring now to FIG. 9, shown therein and designated by a referencenumeral 80 is a second embodiment of a mechanical gain adjustmentassembly constructed in accordance with the present invention. Ingeneral, the mechanical gain adjustment assembly 80 includes a fixedshield 82 which is spaced a distance away from the electrode assembly26, and a movable shield 84. The fixed shield 82 is at least partiallyconstructed of a conductive material and is connected to ground. Thefixed shield 82 has one or more apertures 86 formed therethrough. Themovable shield 84 has one or more apertures 88 formed therethrough. Themovable shield 84 is movable through a predetermined path configured tooverlap the aperture 88 in the movable shield 84 with the aperture 86 infixed shield 82. By varying the amount of overlap between the apertures88 and 86, the covering and uncovering of the electrode assembly 26 iseffected.

In one preferred embodiment, the electrode assembly 26 is provided withan opening 90 formed therethrough. The linkage 42 extends through theopening 90 and supports the movable shield 84. The movement assembly 30in this embodiment is preferably the stepper motor. In this instance,the movement assembly 30 is controlled by the microcontroller 36 to movethe movable shield 84 either to the fully overlapped position, or tosome position that only partially overlaps the apertures 86 and 88 inthe fixed and movable shields 82 and 84. For example, if a stepper motorhaving small steps is used, e.g., 1.8 or 0.9 degrees per step, and ifthe stepper motor is controlled to half-step the stepper motor, veryfine position changes can be made in the overlapping of the apertures 88and 86. In other words, the stepper motor alternately moves the movableshield 84 to a closed or non-overlapped position, and then moves themovable shield 84 to the partially overlapped position so that theelectric-field at the electrode assembly 26 is reduced by somepredetermined fraction that is controlled automatically with everymeasurement cycle. When the movable shield 84 is moved to overlap theapertures 84 and 86 only partially, a reduction in effective aperture isachieved.

As shown in FIG. 9, the mechanical gain adjustment assembly 80 canoptionally be provided with an adjustable shield 92. The adjustableshield 92 is constructed of a conductive material and is connected tothe ground reference potential 42. The adjustable shield 92 has one ormore apertures 94 formed therethrough. The adjustable shield 92 can beadjusted (manually or automated) such that the aperture 94 in theadjustable shield 92 overlaps the aperture 86 in the fixed shield 82,effecting the same purpose as the mechanical gain adjustment assembly50. By varying the amount of overlap between the apertures 94 and 86,the effective aperture is also varied. In other words, the adjustableshield 92 can be moved and fixed to constrict or open the size of theaperture 86 in the fixed shield 82. In one preferred embodiment, theadjustable shield 92 and the fixed shield 82 can be formed of conductivematerial, such as cast metal, or sheet metal that move relative to oneanother.

The size and/or the shape of the apertures 86, 88 and 94 can be varied.The shape of the apertures 86, 88 and 94 can be any of a variety ofgeometric, non-geometric or fanciful shapes. The relative location ofthe movable shield 84, fixed shield 82 and adjustable shield 92 can bevaried.

Although the mechanical gain adjustment assembly 80 has been describedherein as having the movable shield 84, fixed shield 82 and theadjustable shield 92 with the apertures 86, 88 and 94, it should beunderstood that the mechanical gain adjustment assembly 80 should not belimited to the apertures 86, 88 and 94 unless such apertures 86, 88 and94 are specifically recited in the appended claims. The function of theapertures 86, 88 and 94 is to vary the size of the effective aperture.For example, the mechanical gain adjustment assembly 80 can beimplemented with the aperture 86 formed in the fixed shield 82 and themovable shield 84 formed of a vane or a blade movable to completelyoverlap or partially overlap the aperture 86.

Insulated Electrode Assemblies

Referring now to FIGS. 10 and 11, shown therein in greater detail is oneversion of the electrode assembly 26. The electrode assembly 26 isdesigned to permit easy cleaning and/or replacement of the insulatorswith minimal disassembly of the electric-field meter 10. As mentioned inthe background section, it is necessary to clean the insulatorsperiodically when the insulators become covered with films of dust,moisture and salt spray that after a time cause degradation ofinsulation.

The electrode assembly 26 is provided with a sensing electrode 100, astandoff 102, a fixed insulator 104, a replaceable insulator 106, and anelectrode mount 108. The sensing electrode 100 is constructed of aconductive and preferably non-corrosive material, such as stainlesssteel or gold, or plated metal. The fixed insulator 104 is constructedof an insulating material, such as Teflon brand insulator, KEL F brandinsulator, glass or wax. The standoff 102 is constructed of a conductivematerial, such as stainless steel or gold-plated metal. The fixedinsulator 104 is connected to the standoff 102 such that the fixedinsulator 104 extends beyond the periphery of the standoff 102. In oneembodiment, the fixed insulator 104 is connected, e.g., press-fit,threaded, or epoxied, to the standoff 102.

The electrode mount 108 removably connects the sensing electrode 100 tothe standoff 102. The sensing electrode 100 is electrically connected tothe standoff 102. The electrode mount 108 can be a screw, a snapfastener, a friction mount or the like. Although FIG. 10 shows theelectrode mount 108 and the sensing electrode 100 being two separatecomponents which are connected together, it should be understood thatthe electrode mount 108 and the sensing electrode 100 can be of unitaryconstruction. When the electrode mount 108 and the sensing electrode 100are formed of separate components, the electrode mount 108 is desirablyaffixed to the sensing electrode 100 so as to provide a securemechanical and electrical connection therebetween. The replaceableinsulator 106 is positioned between the sensing electrode 100 and thefixed insulator 104. The replaceable insulator 106 can be Teflon brandinsulator, KEL F brand insulator, glass or wax, for example.

To connect the electrode assembly 26 to the electric-field meter 10, thehousing 52 of the electric-field meter 10 is provided with a mountingsurface 114 having an electrode mounting opening 116. The standoff 102and the fixed insulator 104 are disposed in the electrode mountingopening 116. Desirably, the fixed insulator 104 is received within theelectrode mounting opening 116 so as to space the standoff 102 away fromthe mounting surface 114 thereby insulating the standoff 102 from themounting surface 114 of the housing 52.

The electrode assembly 26 can be mounted to the mounting surface 114with any suitable mechanical scheme or linkage. For example, a terminal117 can be mounted to the standoff 102 via a screw 118. In thisinstance, the terminal 117 and the fixed insulator 104 are on opposingsides of the mounting surface 114. The terminal 117 also functions toprovide an electrical connection between the electrode assembly 26 andthe charge measurement circuit 34. Alternatively, the electrode assembly26 can be mounted to the mounting surface 114 by threading the fixedinsulator 104 and the electrode mounting opening 116, or using tabs,adhesive bonding, cohesive bonding, press fitting or the like to securethe fixed insulator 104 to the mounting surface 114.

Once the electrode assembly 26 is mounted to the mounting surface 114,the housing 52 defines a cavity 120 that is sealed. A desiccant pack(not shown) is loaded into the housing 52 before sealing of the cavity120.

To clean or replace the sensing electrode 100, replaceable insulator106, and the fixed insulator 104 of the electrode assembly 26, theelectrode mount 108 is manipulated from outside of the housing 52 so asto remove the sensing electrode 100 and the replaceable insulator 106from the standoff 102. For example, when the electrode mount is a screw,the screw can be removed with a screwdriver. Once the sensing electrode100 and the replaceable insulator 106 are removed from the standoff 102,the electrode 100 and the replaceable insulator 106 can be cleaned, orreplaced and the fixed insulator 104 can be cleaned. Thus, it will beunderstood by those skilled in the art that the sensing electrode 100and the replaceable insulator 106 can be removed and/or cleaned withminimal disassembly of the electric-field meter 10. This permitsreplacement or cleaning of the sensing electrode 100 and the replaceableinsulator 106 in the field by a relatively unskilled technician. This isa vast improvement over the prior art electric-field meters whichrequired an extensive disassembly of the electric-field meter to cleanand/or replace the electrodes and/or insulators.

Position Detection Assemblies

Referring now to FIG. 12, shown therein in schematic or block diagramform is the position detection assembly 32. In every type ofelectro-static field measuring machine, the position of the movingconductor, e.g., the shield assembly 28 of a field mill, must be knownin order to determine the polarity of the field being measured and toknow the time at which the signal is at peak value. In theelectric-field meter 10, when the movement assembly 30 is a steppermotor the position of the shield assembly 28 is known automatically andintrinsically as a result of the microcontroller 36 controlling thestepper motor as discussed above. However, even with the movementassembly 30 being the stepper motor, position errors such as misstepscan occur. For this reason, the position of the shield assembly 28 ispreferably detected directly by the position detection assembly 32,rather than by using additional components that must be referenced tothe shield assembly 28 by subsequent mounting and adjustment.

In general, the position detection assembly 32 is provided with a firstelement 130 mounted onto one or more vane 29 of the shield assembly 28,a second element 132 (FIGS. 11 and 12) mounted in a known relationshipto the predetermined path of the shield assembly 28, and a detectcircuit 134 receiving an output from the first element 130 or the secondelement 132. The first element 130 is epoxied, soldered, riveted,bolted, spot welded, threaded, or otherwise mechanically attached to theshield assembly 28. The second element 132 is desirably mounted adjacentto the mounting surface 114 of the housing 52 such that the shieldassembly 28 and the second element 132 are mounted on opposing sides ofthe mounting surface 114. However, it should be understood that thefirst and second elements 130 and 132 can be mounted on the same side ofthe mounting surface 114. The second element 132 detects the movement ofthe first element 130 when the first element 130 passes near the secondelement 132, and in response thereto outputs one or more signals insynchrony with the position of the shield assembly 28 over the electrodeassembly 26. The detect circuit 134 receives the signals from the firstelement 130 or the second element 132 and in response thereto conditionssuch signals to be in a form recognizable by the microcontroller 36.

In a preferred embodiment shown in FIGS. 11 and 12, the first element130 is a magnet, the second element 132 is a coil and the detect circuit134 is a comparator. This embodiment requires only a low-powercomparator to perform pulse detection and minimal low-powercombinatorial logic as active components, therefore enabling synchronousshield assembly 28 detection at microwatt power levels with noadjustment or calibration required. The coil is desirably located in aco-axial relationship with the sensing electrode 100. Alternatively, thefirst or second elements 130 and 132 can include, for example,modulation of the self-inductance of a coil that is used as an elementof an oscillator by sweeping a ferrous object near the coil, opticalcomponents and a light source, printing or etching to provide variationsin reflected light to an appropriately mounted sensor, solid statemagnetic field sensors such as Hall Effect devices, SQUIDS, etc. Forexample, a light source and light detector (first element 130) can beattached to the vane 29 of the shield assembly 28, and a retroreflector(second element 132) can be attached to the mounting surface 114.Alternatively, the retroreflector (first element 132) can be attached tothe vane 29 of the shield assembly 28, and the light source and lightdetector (second element) can be attached to the mounting surface 114.

Electrical Contact Assembly

In accordance with the present invention, the electric-field meter 10 isprovided with an electrical contact assembly 140 (FIGS. 13 and 14) forgrounding or receiving a signal from a moving electrode assembly 26, orthe shield assembly 28. In one preferred embodiment, the electricalcontact assembly 140 engages a moving conductor 142, which is typicallythe linkage 42 of the movement assembly 30. For example, when theelectric-field meter 10 is configured as a field mill, the electricalcontact assembly 140 grounds the shield assembly 28, whereas when theelectric-field meter 10 is configured as an agrimeter, the electricalcontact assembly 140 receives a signal from the electrode assembly 26 c.

Shown in FIG. 13 is an embodiment of the electrical contact assembly 140for use in applications where the movement assembly 30 moves the shieldassembly 28 a in rotating, reciprocating or oscillating fashion. Theelectrical contact assembly 140 engages the linkage 42 of the movementassembly 30 and thereby provides electrical contact therebetween. Theelectrical contact assembly 140 is provided with a pair of conductingmembers 144 a and 144 b. The conducting members 144 a and 144 b arespaced a distance apart. The conducting members 144 a and 144 b areelectrically connected to and supported by a standoff 146. The standoff146 is connected to the mounting surface 114 of the electric-field meter10, or frame of the electric-field meter 10 to provide support for theconducting members 144 a and 144 b. Each of the conducting members 144 aand 144 b supports a pad 148 a and 148 b, respectively, engaging thesides or an end of the linkage 42. The pads 148 a and 148 b can beconstructed of a conductive cloth material, such as carbonized cloth,graphite cloth, metal cloth or the like.

To provide low friction, low noise, electrical contact between theconducting members 144 a and 144 b and the linkage 42, each of the pads148 a and 148 b is loaded with a conductive lubricant, such as Nyogel753G obtainable from Nye, other conductive particles, or oils or greasesloaded with conductive particles. The conducting members 144 a and 144 bcan be connected to the ground reference potential 40 to ground thelinkage 42. Alternatively, the conducting members 144 a and 144 b can beelectrically connected to the charge measurement circuit 34 so that thecharge measurement circuit 34 receives a signal from a moving electrode.

Shown in FIG. 15 and designated by a reference numeral is anotherexample of an electrical contact assembly 150 constructed in accordancewith the present invention for use in applications where the movementassembly 30 moves the shield assembly 28 in a rotating, reciprocating oroscillating fashion. The electrical contact assembly 150 is providedwith a sealed antifriction bearing 151 having an inner surface 152, anouter surface 154, one or more rolling element(s) 156, and a cage 158for retaining the rolling element(s) 156 in position. The surfaces 152and 154 are typically designed to reduce (as far as possible) allfriction. However, the surfaces 152 and 154 could be formed on abushing. The cage 158 retains the rolling element(s) 156 in a spacedapart position between the inner surface 152 and the outer surface 154.The rolling elements 156 a can be spherical balls, or rollers. Therollers can be provided with any suitable shape, such as cylindrical,tapered, barrel-shaped, needle-shaped or the like.

The outer surface 154 and the inner surface 152 are constructed ofconductive material(s), such as stainless steel, aluminum or brass. Theouter surface 154 is typically supported by the mounting surface 114,frame or other bodies of metal in the electric-field meter 10. When itis desired to ground the shield assembly 28, the mounting surface 114,frame or other bodies of metal in the electric-field meter 10 can beconnected to ground, e.g., in a field mill.

When it is desired to receive a signal from a moving electrode (e.g., anagrimeter), a connector can be connected to the mounting surface 114,frame or other bodies of metal in the electric-field meter 10 forreceiving the signal. The inner surface 152 engages the linkage 42.

The rolling elements 156 are spaced a distance apart to form at leastone void therebetween. To lubricate the rolling elements 156 a as wellas increase the conductivity between the outer surface 154 and the innersurface 152, the anti-friction bearing includes conductive lubricantpositioned about the rolling elements 156 a and within the void. Theconductive lubricant can be Nyogel 753G obtainable from Nye or any of avariety of grease-like or lubricating substances that are loaded withconductive particles. The conductive lubricant is sealed within the voidof the anti-friction bearing 151 by any suitable sealing device, such asgrease grooves in the bore of the housing, felt washers, leather orsynthetic-rubber seals, labyrinth washers or the like.

The construction and use of bearings is well known in the art. Thus, nofurther comments are deemed necessary to teach one skilled in the art tomake and use the electrical contact assembly 150 of the presentinvention.

Shown in FIGS. 16 and 17 are two other examples of an electrical contactassembly 160 constructed in accordance with the present invention foruse in applications where the movement assembly 30 moves the shieldassembly 28 or the electrode assembly 26 in the reciprocating oroscillating manner. The electrical contact assembly 160 includes aflexible conductor 161 bonded to the linkage 42 and bonded either to theground reference potential 40 or the charge measurement circuit 34. Thiscompletely obviates the need for any sort of wiping or probing contactand therefore eliminates the limitations imposed by such means. Theflexible conductor 161 is arranged to have sufficient freedom ofmovement so that it can perform a large number, such as billions ofcycles without breaking or losing contact. The flexible conductor 161can be bonded to an end or side 162 of the linkage 42 in variousmanners. For example, the flexible conductor 161 can be clamped,crimped, screwed, soldered, or welded to the linkage 42. The flexibleconductor 161 can be implemented as a solid wire (as shown in FIG. 16),a stranded wire, a coiled metal spring (as shown in FIG. 17), a flexiblesheet metal strip or the like.

One of the electrical contact assemblies 140, 150 or 160 can be used forgrounding the shield assembly 28 or receiving a signal from a movingelectrode, or more than one of the rotor contact assemblies 140, 150 or160 can be used to provide redundancy and thereby increase thereliability of the electric-field meters 10, 10 a, 10 b and 10 c.Alternatively, combinations of the electrical contact assemblies 140,150 and 160 can be used to provide redundancy.

Error Detection and Correction

As discussed above in the Background section, prior-art field meterssuffer from two types of uncorrected errors that change with time,temperature, humidity and atmospheric pollutants. Typical instrumentsthat predate the present invention have a zero-signal output (defined asthe output value of the field meter with an imposed electric field ofzero) that is typically set during manufacture but which subsequentlychanges in an unknown way with use and time. Because valuableinformation about atmospheric electrical conditions can be obtainedaround zero and at the zero-crossing, i.e., when the electric fieldreverses polarity, there is a significant advantage in having azero-signal reading that is known with confidence throughout theoperating life of the instrument.

Prior-art field meters also suffer from variations in leakage current atthe charge-amplifier input due to conduction across insulatorsassociated with the sense electrode and the circuitry used for chargemeasurement. For prior-art field meters at the place and time ofmanufacture, the average leakage current at the charge-amplifier inputis typically negligible but can change in magnitude and/or polaritydepending upon insulator cleanliness, and offset voltage of the chargeamplifier. The average leakage current in prior-art field meters is anunknown variable that can degrade an instrument to a state of improperoperation without warning. Uncorrected changes in average leakagecurrents tend to cause errors in the measured electric field which canlead to improper assessment of atmospheric electrical threats.

Field meters that suffer from unknown and uncorrected zero-signal offseterrors and average leakage currents at the charge amplifier input do notalways provide information of high quality over long periods of use andsuch field meters typically require labor intensive testing, adjustingand cleaning at times that have to be determined empirically. Here weteach methods for making field meters that measure and correctzero-signal offset errors and errors due to leakage current at thecharge-amplifier input of the charge measurement circuit 34automatically and continuously so as to enhance the quality of themeasurements made. The zero-signal offset errors can be determinedduring each measurement cycle, or as frequently as desired. For example,the zero-signal offset errors can be determined during every measurementcycle, every third measurement cycle, or every tenth measurement cycle.

Field meters of the present invention can also be preset, at the factoryor by the user, to warn in some way of increases in the average leakagecurrent at the charge-amplifier input of the charge measurement circuit34 allowing the instrument to inform users when maintenance, e.g.,insulator cleaning or replacement, is required, rather than relying onscheduled maintenance or waiting for malfunction. These error detecting,correcting and monitoring features can be used automatically or manuallyto prevent use of degraded measurements.

Specific features of the present invention permit these novel errordetection and correction capabilities when combined with measurementsteps taken in sequence by the microcontroller 36. Zero-signal output istested at least twice per measurement cycle when the shield assembly 28is in the position that completely covers the sense electrode of theelectrode assembly 26. As shown in FIG. 12, the charge measurementcircuit 34 includes a charge-amplifier, i.e., an operational amplifier170 configured as a charge-to-voltage transducer, with a capacitor C asthe gain determining element and a selectable resistance R across thecapacitor C that sets the decay rate of charge on the capacitor, allowsoptimum setting of circuit parameters for error detection purposes andnormal measurements.

The selectable resistance R can be a low value, e.g., the on-resistanceof a relay, to quickly reset the charge amplifier, an intermediate valueto provide a fixed decay rate throughout the measurement, or be removedfrom the circuit to provide the longest time constant possible for thecharge amplifier circuit. Mechanical switches or relays can be employedto implement the selectable resistance R. Alternatively, the selectableresistance R may be implemented with switchable, fixed resistors, amotor-driven variable resistor, or a solid-state voltage-controlledresistor such as a field-effect transistor. These various methods ofimplementing a selectable resistance are well known in the art. Theresistance value of the selectable resistance R is selected by themicro-controller 26 via a signal path 172.

The error detecting, correcting and monitoring features of the presentinvention may be implemented with field meters 10, 10 a, 10 b and 10 cof all types including field meters 10, 10 a, 10 b and 10 c that userotary, reciprocating or oscillating motion of one conducting elementrelative to another conducting element, providing that properconsiderations for motion control, moving conductor (i.e., shieldassembly 28 or electrode assembly 26) position detection, etc., aretaken into account.

The steps taken by the microcontroller 36 to measure and correct forzero-signal offset error are:

-   -   a. Position the movable shield assembly 26 (or the electrode        assembly 26) so that the sense electrode 100 is completely        shielded from the external electric field.    -   b. Select a resistance across the capacitor C in the        charge-amplifier that gives a short decay-time typically less        than 5 milliseconds of the RC circuit in the charge-amplifier.    -   c. Select a resistance across the capacitor C in the        charge-amplifier that gives a long decay-time typically at least        1 second of the RC circuit in the charge-amplifier.    -   d. Allow for settling time and digitize the zero-signal offset        error (hereafter referred to as Verror) and store the        measurement.    -   e. Position the movable shield assembly 26 to selectively expose        the sense electrode to the external electric field.    -   f. Digitize the charge-amplifier output (hereafter referred to        as Vsig).    -   g. Compare the previously stored zero-signal offset error and        the normal measurement using mathematical operations in such a        way that the zero-signal offset error contribution to the        overall measurement is removed. For example, subtraction can be        used (Vsig−Verror) to remove the zero-signal error.    -   h. Store and/or communicate the corrected measurement of        electric field to the outside world.

The steps above for determining the zero-signal offset error andcorrecting for the zero-signal offset error can be taken at any desiredrate including but not limited to one or more times per measurementcycle or at any desired rate. Moreover, the correction for thezero-signal offset error can be accomplished locally or at a remotesite.

The steps taken to measure and correct for the average leakage currentat the charge-amplifier input of the charge measurement circuit 34 are:

-   -   a. Position the movable shield assembly 26 (or the electrode        assembly 26) so that the sense electrode 100 is completely        shielded from the external electric field.    -   b. Select a resistance across the capacitor C in the        charge-amplifier that gives a short decay-time typically less        than 5 milliseconds of the RC circuit in the charge-amplifier.    -   c. Select a resistance across the capacitor C in the        charge-amplifier that gives a long decay-time typically at least        1 second of the RC circuit in the charge-amplifier.    -   d. Allow for settling time and digitize the zero-signal offset        error (hereafter referred to as Voffset1) and store the        measurement.    -   e. Wait a sufficient time (eg. 10 ms) for a non-negligible        leakage current to cause a change in the zero-signal offset        error.    -   f. Position movable shield assembly 26 so that the sense        electrode 100 is again completely shielded from the external        electric field.    -   g. Allow for settling time and digitize the zero-signal offset        error (hereafter referred to as Voffset2) and store the        measurement.    -   h. Compare Voffset1 and Voffset2 in such a way that the average        leakage current is determined. For example, the average leakage        current is equal to C*(Voffset2−Voffset1)/(Toffset2−Toffset1).    -   i. Store and/or communicate the average leakage current        associated with the measurement of the electric field to the        outside world.    -   j. Utilize the new, measured average leakage current for        compensation of the following measurement.

The steps above can be taken at any desired rate including but notlimited to one or more times per measurement cycle. Moreover, thecorrection for the average leakage current can be accomplished locallyby the microcontroller 36, for example, or at a remote site. In theinstance where the correction for the average leakage current and/or thezero-signal offset error is accomplished at the remote site, allmeasured and stored information would be output to the remote site bythe field meter 10. Furthermore, although the algorithms or steps takenfor calculating the average leakage current and the zero-signal offseterror are discussed above separately, it should be understood that inpractice these steps could be combined into a single algorithm.

The selectable resistor R can be replaced with a fixed resistor having ahigh value to provide the RC circuit with a long-time constant, such asgreater than 1 second. In this instance, one must wait until thecapacitor is discharged before determining the zero-signal offset error.

As discussed above, the steps taken to measure and correct for theaverage leakage current at the charge-amplifier input of the chargemeasurement circuit 34 includes making two zero signal offsetmeasurements separated sufficiently in time. The average leakage currentis then determined from the the average rate-of-change of the zerosignal offset of the charge amplifier. Differences between these zerosignal values are primarily due to the leakage current on thecharge-amplifier input of the charge measurement circuit 34. As such,increasing leakage currents can result in increasing differences betweenthe zero signal values and eventually result in measurement over-range.To significantly extend the useful range of leakage currents in whichthe steps taken to measure and correct for the average leakage currentat the charge-amplifier input of the charge measurement circuit 34 canfunction optimally, the charge measurement circuit 34 can furtherinclude a leakage current compensation circuit 200, such as shown forexample in FIG. 12.

In general, the current compensation circuit 200 automatically andcontinuously (i.e., in real-time) generates a compensation currentgenerally equal to and opposite in polarity to the unwanted leakagecurrent on the charge-amplifier input of the charge measurement circuit34. In one preferred embodiment, the compensation current is based onthe measured average leakage current utilized in the steps taken by themicrocontroller 36 to measure and correct for the average leakagecurrent at the charge-amplifier input of the charge measurement circuit34 (discussed above).

As shown in FIG. 12, in one embodiment, the leakage current compensationcircuit 200 includes a compensation voltage source 204 which generates aprogrammable compensation output (hereafter referred to as Vcomp). TheVcomp is connected to the charge-amplifier input by a large valueresistance Rcomp, in which the desired compensation current (hereafterreferred to as Icomp) is developed. In one preferred embodiment, thecompensation voltage source 204 is a digital-to-analog converter 206controlled by the microcontroller 36. Further, the range and resolutionof the leakage current compensation circuit 200 can be adjusted byvarying the Vcomp voltage (e.g. via the D/A convertor 206 andmicrocontroller 36) and/or the magnitude of the resistance of Rcomp(e.g. via a variable or selectable resistor).

Any method that can generate a known average current can be used togenerate a leakage current compensation. For example, a multiplexermultiplexing four voltages across a resistor could be used.

Changes may be made in the construction and operation of the variouscomponents, elements and assemblies described herein and changes may bemade in the steps or the sequence of steps of the methods describedherein without departing from the spirit and the scope of the inventionas defined in the following claims.

1. An electric-field meter f or measuring at least one of a magnitudeand polarity of an electric field, comprising: a housing at leastpartially constructed of a conductive material, the housing defining aretaining space; an electrode assembly on the housing and selectivelyexposed to the electric field; a shield assembly for alternatelycovering and exposing the electrode assembly to the electric field; amovement assembly having a source of motive force and a linkage operablyconnected to one of the shield assembly and the electrode assembly foralternately covering and exposing the electrode assembly to the electricfield; a position detection assembly for monitoring the position of atleast one of the shield assembly and the electrode assembly andproviding a position detection signal indicative of the position of oneof the shield assembly and the electrode assembly; a charge measurementcircuit having an input receiving charge on the electrode assembly, thecharge measurement circuit providing a charge detection signalindicative of the charge induced on the electrode assembly as theelectrode assembly is selectively exposed to the electric field; meansfor determining an average leakage current at the input of the chargemeasurement circuit; and means for generating a compensation currentgenerally equal to and opposite in polarity to the determined averageleakage current at the input of the charge measurement circuit, whereinthe compensation current is supplied to the input of the chargemeasurement circuit.
 2. The electric-field meter of claim 1, furthercomprising means for determining a zero-signal offset error.
 3. Theelectric-field meter of claim 1, further comprising means for correctingfor the average leakage current at the input of the charge measurementcircuit.
 4. The electric-field meter of claim 3, further comprisingmeans for determining and correcting for a zero-signal offset error. 5.The electric-field meter of claim 1, wherein the means for generating acompensation current comprises: a compensation voltage source generatinga programmable compensation output; and a resistance in which thecompensation current is developed.
 6. The electric-field meter of claim5, wherein the compensation voltage source is a digital-to-analogconverter controlled by the means for determining an average leakagecurrent at the input of the charge measurement circuit.
 7. Theelectric-field meter of claim 1, further comprising a flexible conductorbonded to the linkage of the movement assembly for maintainingelectrical contact between the movement assembly and at least one of theground reference potential and the charge measurement circuit.
 8. Anelectric-field meter for measuring at least one of a magnitude andpolarity of an electric field, comprising: a housing at least partiallyconstructed of a conductive material, the housing defining a retainingspace; an electrode assembly on the housing and selectively exposed tothe electric field; a shield assembly for alternately covering andexposing the electrode assembly to the electric field; a movementassembly having a source of motive force and a linkage operablyconnected to one of the shield assembly and the electrode assembly foralternately covering and exposing the electrode assembly to the electricfield, the movement assembly being stationary and exposing at least aportion of the electrode assembly for a predetermined time periodwhereby the electric-field meter functions as an electric-field-changemeter during the predetermined time period; a charge measurement circuithaving an input receiving charge on the electrode assembly, the chargemeasurement circuit providing a charge detection signal indicative ofthe charge induced on the electrode assembly as the electrode assemblyis selectively exposed to the electric field; means for determining anaverage leakage current at the input of the charge measurement circuit;and means for generating a compensation current generally equal to andopposite in polarity to the determined average leakage current at theinput of the charge measurement circuit, wherein the compensationcurrent is supplied to the input of the charge measurement circuit. 9.The electric-field meter of claim 8, further comprising means fordetermining a zero-signal offset error.
 10. The electric-field meter ofclaim 8, further comprising means for correcting for the average leakagecurrent at the input of the charge measurement circuit.
 11. Theelectric-field meter of claim 10, further comprising means fordetermining and correcting for a zero-signal offset error.
 12. Theelectric-field meter of claim 8, wherein means for generating acompensation current comprises: a compensation voltage source generatinga programmable compensation output; and a resistance in which thecompensation current is developed.
 13. The electric-field meter of claim12, wherein the compensation voltage source is a digital-to-analogconverter controlled by the means for determining an average leakagecurrent at the input of the charge measurement circuit.
 14. Theelectric-field meter of claim 8, further comprising a flexible conductorbonded to the linkage of the movement assembly for maintainingelectrical contact between the movement assembly and at least one of theground reference potential and the charge measurement circuit.
 15. Anelectric-field meter for measuring at least one of a magnitude andpolarity of an electric field, comprising: a housing at least partiallyconstructed of a conductive material, the housing defining a retainingspace; an electrode assembly on the housing and selectively exposed tothe electric field; a shield assembly for alternately covering andexposing the electrode assembly to the electric field; a movementassembly having a source of motive force and a linkage operablyconnected to one of the shield assembly and the electrode assembly foralternately covering and exposing the electrode assembly to the electricfield; a position detection assembly for monitoring the position of atleast one of the shield assembly and the electrode assembly andproviding a position detection signal indicative of the position of oneof the shield assembly and the electrode assembly; a charge measurementcircuit having an input receiving charge on the electrode assembly, thecharge measurement circuit providing a charge detection signalindicative of the charge induced on the electrode assembly as theelectrode assembly is selectively exposed to the electric field; andmeans for automatically and continuously determining an average leakagecurrent at the input of the charge measurement circuit.
 16. Theelectric-field meter of claim 15, further comprising means forautomatically and continuously determining a zero-signal offset error.17. The electric-field meter of claim 15, further comprising means forautomatically and continuously correcting for the average leakagecurrent at the input of the charge measurement circuit.
 18. Theelectric-field meter of claim 17, further comprising means forautomatically and continuously determining and correcting for azero-signal offset error.
 19. An electric-field meter for measuring atleast one of a magnitude and polarity of an electric field, comprising:a housing at least partially constructed of a conductive material, thehousing defining a retaining space; an electrode assembly on the housingand selectively exposed to the electric field; a shield assembly foralternately covering and exposing the electrode assembly to the electricfield; a movement assembly having a source of motive force and a linkageoperably connected to one of the shield assembly and the electrodeassembly for alternately covering and exposing the electrode assembly tothe electric field; a position detection assembly for monitoring theposition of at least one of the shield assembly and the electrodeassembly and providing a position detection signal indicative of theposition of one of the shield assembly and the electrode assembly; acharge measurement circuit receiving charge on the electrode assembly,the charge measurement circuit providing a charge detection signalindicative of the charge induced on the electrode assembly as theelectrode assembly is selectively exposed to the electric field; andmeans for automatically and continuously determining a zero- signaloffset error.
 20. The electric-field meter of claim 19, furthercomprising means for automatically and continuously correcting for thezero-signal offset error.